In the machining of workpieces with laser beams, for example in laser cutting, laser welding, laser inscribing or laser engraving, machining heads are used, the machining heads can be moved with respect to the workpiece that is to be machined, and in them, a laser beam emitted from a machining laser is guided by means of an optical system onto the surface of the workpiece and focused there at a machining point. Exact positioning and focusing of the laser beam of the machining laser on the surface of the workpiece is necessary to guarantee as efficient as possible a conduct of the laser beam machining. In this regard, for example, a method and a device for detecting and setting the position of the laser beam on the surface of a workpiece being machined is known from EP 1 908 544 A2. With the known method, first, the actual values of the position of the laser beam are detected via one or more beam position detectors and are then matched with the specified values of the laser position by adjusting the optical system. The adjustment of the optical system in this case takes place, for example, via a mirror arrangement with an adaptive mirror.
In addition, methods for setting the focal position of a laser beam of a machining laser aimed at a workpiece are known from the prior art. For example, a method for setting the focal position of a laser beam aimed at a workpiece is described in DE 102 48 458 B4, where the distance between the machining head from which the laser beam comes and the surface of the workpiece is kept constant, and the proportion of radiation coming from a region of the zone of interaction between the laser beam and workpiece is detected within the machining head by a radiation detector and focusing optics disposed in the machining head are shifted so that a signal corresponding to the detected radiation takes on a maximum value. The maximum value of the detected radiation power is achieved when the focal position of the laser beam relative to the workpiece is optimum for the machining. The focal position of the laser beam of the machining laser relative to the workpiece and to the machining head can be established with high precision with this method, and the establishment of the focal position initially takes place “off line” before the actual machining operation.
However, problematic with this method is the change of the focal position of the machining beam during the machining process under the thermal load of the power of the laser beam and thus the change of the effective machining point (tool center point, TCP). Changes of the position of the TCP arise as a consequence of thermal stress of the optical components of the beam guiding system and due to thermally caused focal length changes (“thermal lensing” effects). To conduct a machining process that is as efficient as possible, these thermal effects, which lead to a change of the position of the effective machining point on or with respect to the surface of the workpiece being machined, must be accounted for in establishing the focus of the laser beam of the machining laser.